In recent years, inorganic semiconductor nanocrystals whose size and shape dispersion are very small have been able to be prepared as a colloidal dispersion in solvent. Monodisperse semiconductor fine crystals having a size of about several nanometers are precipitated from a solution phase and so-called “semiconductor quantum dots” are obtained in the form of a colloidally dispersed solution thereof. In most cases, the surface of a semiconductor fine crystal, which is regarded as a singe crystal, is terminated with organic molecule ligands to provide a semiconductor nanocrystal as a quantum well, thereby forming a three-dimensional quantum well (quantum dot) wherein the organic ligand and solvent serve as a potential barrier. Alternatively, the semiconductor nanocrystal (corresponding to core) is surrounded thereabout with a wide-gap semiconductor layer (corresponding to a shell) to form so-called “type I quantum well structure.” Where the size of the semiconductor fine crystal is sufficiently smaller than the Bohr radii of the electron and hole within the semiconductor, the crystal is surrounded thereabout with a potential barrier having a satisfactory height and thickness, thereby enabling discrete quantized levels to be formed. When a transition probability toward the outside of the semiconductor becomes small, the electron and hole excited to the quantized levels are mutually re-combined, so that photoluminescence is very efficiently emitted even at room temperature. Especially, it has been found that within quantum dots constituted of CdSe, CdS, PbSe, HgTe, CdTe, InP, GaP, InGaP, GaAs, InGaN, GaN or the like, or a mixed crystal thereof, the Wannier-Mott exciton wherein a confined electron and hole mutually form a bound state to form a hydrogen type orbit stably exists at room temperature in a nanocrystal having a diameter of about 2 to 6 nm. In this case, the exciton is confined in a space much smaller than the Bohr radius in free states and thus, the binding energy becomes drastically large. By acquiring such a binding energy that is adequately larger than a thermal energy in a room temperature environment, the dot functions as a radiative center stably in a high efficiency during the course of light emission.
During several years in the past, there have been proposed devices incorporating an organic polymer matrix and inorganic semiconductor nanocrystals into a structure analogous to a direct current drive diode using an organic phosphor, such as aluminium quinolinol (Alq3) or the like as a light emitting material, i.e. an organic electroluminescent (EL) device (OLED) (Non-Patent Documents 1 to 3) wherein using the affinity of molecular orbit levels, carrier injection, and transport and light emission are carried out, and light emission phenomena thereof have been reported (Non-Patent Documents 4 to 12).
Patent Document 1: WO2006/043656
Patent Document 2: Japanese Patent Laid-open No. 2003-173878
Patent Document 3: Japanese Patent Laid-open No. 2003-249373
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